linux-kernel - Re: [PATCH] fs/pipe: use kvcalloc to allocate a pipe

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Date:   Wed, 5 Jan 2022 06:54:20 +0000
From:   Dmitry Safonov <0x7f454c46@...il.com>
To:     Andrei Vagin <avagin@...il.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] fs/pipe: use kvcalloc to allocate a pipe_buffer array

On 1/4/22 17:10, Andrei Vagin wrote:
> Right now, kcalloc is used to allocate a pipe_buffer array.  The size of
> the pipe_buffer struct is 40 bytes. kcalloc allows allocating reliably
> chunks with sizes less or equal to PAGE_ALLOC_COSTLY_ORDER (3). It means
> that the maximum pipe size is 3.2MB in this case.
> 
> In CRIU, we use pipes to dump processes memory. CRIU freezes a target
> process, injects a parasite code into it and then this code splices
> memory into pipes. If a maximum pipe size is small, we need to
> do many iterations or create many pipes.
> 
> kvcalloc attempt to allocate physically contiguous memory, but upon
> failure, fall back to non-contiguous (vmalloc) allocation and so it
> isn't limited by PAGE_ALLOC_COSTLY_ORDER.
> 
> The maximum pipe size for non-root users is limited by
> the /proc/sys/fs/pipe-max-size sysctl that is 1MB by default, so only
> the root user will be able to trigger vmalloc allocations.
> 
> Cc: Dmitry Safonov <0x7f454c46@...il.com>
> Signed-off-by: Andrei Vagin <avagin@...il.com>

Good idea!

I wonder if you need to apply this on the top:

diff --git a/fs/pipe.c b/fs/pipe.c
index 45565773ec33..b4ccafffa350 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -605,7 +605,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 static long pipe_ioctl(struct file *filp, unsigned int cmd, unsigned
long arg)
 {
        struct pipe_inode_info *pipe = filp->private_data;
-       int count, head, tail, mask;
+       unsigned int count, head, tail, mask;

        switch (cmd) {
        case FIONREAD:
@@ -827,7 +827,7 @@ struct pipe_inode_info *alloc_pipe_info(void)

 void free_pipe_info(struct pipe_inode_info *pipe)
 {
-       int i;
+       unsigned int i;

 #ifdef CONFIG_WATCH_QUEUE
        if (pipe->watch_queue) {
--->8---

Otherwise this loop in free_pipe_info() may become lockup on some ugly
platforms with INTMAX allocation reachable, I think. I may be wrong :-)

Thanks,
          Dmitry